Potomac Horse Fever (PHF), an acute infectious disease of horses, was reportedly first recognized in 1979 in the region of the Potomac River in Maryland and Virginia. The disease is also identified as Equine Monocytic Ehrlichiosis and Equine Intestinal Ehrlichiosis. The causative agent is a gram-negative, obligate intracellular bacterium which was first identified in 1984 as Ehrlichia risticii (E. risticii), but has been renamed Neorickettsia risticii (N. risticii). The disease is characterized by a wide variety of intestinal symptoms, along with elevated temperature and inflamed mucus membranes. In many cases it results in severe pain, and sometimes death.
The bacteria infect the enterocytes of the small and large intestine, resulting in acute colitis syndrome, and producing symptoms of mild colic, fever, depression, anorexia and diarrhea in horses of all ages. The disease can also cause abortion in pregnant mares, laminitis, and death. N. risticii has been isolated from trematodes infecting fresh water snails, and from caddisflies, mayflies, damselflies, dragonflies and stoneflies. The route of infection appears to be inadvertent ingestion of the aquatic insects carrying N. risticii, and the incubation period is 10 to 18 days.
An antigen for use in an assay to detect the presence of N. risticii has been claimed in U.S. Pat. No. 4,759,927, which is herein incorporated by reference in its entirety. Its source was later identified as the 25-D strain. A second strain identified as 90-12, was disclosed in U.S. Pat. No. 6,375,954, which is herein incorporated by reference in its entirety. U.S. Pat. No. 6,375,954 claims a method for protecting against N. risticii by administering a particular 90-12 protein antigen. Equine vaccines are commercially available, but provide only partial or no protection against newer strains of N. risticii, and against strains we have isolated relating to the present invention.
The citation of any reference herein should not be construed as an admission that such reference is available as “prior art” to the instant application.